This addon clears your YT "Watch Later" playlist.
You need to be logged in to your YT account. Then navigate to "watch later" playlist.
Paste the URL from the addons page.
Here it is:
https://outgoing.prod.mozaws.net/v1/2a8a819bf623e4dcf45b496067eefe2fc0933bc356f5e85143f29c82654bd7c5/https%3A//www.youtube.com/playlist%3Flist=WL&disable_polymer=true
Now click on the YT icon at top right in your addons bar.
A dialog shows up prompting you, just click OK.

Big thanks to the developer for making this addon! =)

Just go to your playlist home page (while logged...)
and press the "Clear Watch Later Videos" extension button. It will delete videos from the list one by one as if you were doing it by hand. This is a smart way to do it as YouTube doesn't want you to have full control of your Watch Later list. So press the button and wait a little while for it to finish deleting all of the videos (I had 651 of them...). Thanks, developer!!

Pretty neat, don't have to paste the script each time.